The present invention refers to the medical field. More in detail the present invention refers to a new prosthetic valve to be implanted in the venous system, typically of the lower limbs, adapted to allow a physiological blood flow where such flow has been altered by diseases causing a reflux affecting the lower limbs. The venous system of the lower limbs represents a complex system, constituted by a deep district formed by the satellite veins of the arteries, a superficial system represented by the saphenae and their branches, a system of interconnection between the two constituted by the perforating veins and by a rich network of intramuscular veins that are particularly important with regard to the rear leg muscles. Each of these systems can generate physiopathological alterations such to determine a significant chronic venous insufficiency scenario.
The disease of the deep venous system is particularly serious, both with regard to difficult treatment and the fact that actions taken to confront associated superficial system and perforating vein diseases have extremely temporary effect. The venous system of the lower limbs in fact acts as a single entity, and the alteration of one sector is generally transmitted to the others. In particular, it is known that large-size varices can cause an expansion of the deep venous system with relative incompetence of the valves thereof, returning to size and function only after correction of the varices themselves. In other words, this is a reversible functional insufficiency of the deep venous system due to an overload by the superficial system. The reverse interconnection is the expansion of the superficial system due to a deep reflux through the expansion of the perforating veins. The ligature of the latter leads to temporary results, since the disease of the depth is capable of continuously expanding other perforating veins.
Hence, the reflux disease affecting the lower limbs is to be examined by investigating the entire venous system and the treatment aimed towards correcting the deep system, if pathological. In general, a reflux affecting the deep venous system originates from alterations of the structure which physiologically prevents reflux, i.e. the valve. Valves are either congenitally altered or they have acquired such alteration. Congenital alteration can consist of poor functioning, as an altered structure, or absence of the valve itself: poor functioning is the most common form. Given that the valve is constituted by two intimal flaps arranged with their opening upward, in a manner so as to form sacks such to prevent the anti-physiological flow of the venous flow, it may happen that these flaps do not carry out their containment function and allow the blood flow in both directions. This possibility is termed primary valve insufficiency. In other occasions, the flaps are present but hypotrophic; in other cases, much rarer, they are atrophic and hence entirely absent. These scenarios generally determine a serious form of chronic venous insufficiency, already present in youth, and in addition they can be complicated during life with the superimposition of phlebothrombosis. The alteration of the valves of acquired type instead derives from the lesion thereof by a thrombotic process, and therefore this is part of the post-thrombotic syndrome. The post-thrombotic syndrome, much more frequent than congenital forms, originates from the rechanneling by endogenous fibrinolysis processes with extended phlebothrombosis affecting the deep venous system. The veins can reacquire their axial permeability but generally the valves result completely damaged and incorporated in a parietal thickening process of fibrotic type. The size of the reflux, generally associated with alteration of the superficial venous system and of the perforating veins, generates scenarios of severe chronic venous insufficiency, with appearance of relapsing ulcers. When a chronic venous insufficiency is controllable with operations affecting the superficial venous system or corrections of perforating veins associated with physical aids (elastic sock), the presence of deep venous reflux can be tolerated. In specific cases, nevertheless, significant and disabling symptoms can suggest a surgical operation aimed for correcting the deep reflux itself. The importance of deep venous reflux in the genesis of alterations of trophism of the limb was already known to Linton in 1931, who suggested as correction the ligature of the femoral vein, thus introducing the concept that an obstructed flow was preferable to the reflux itself. Along the same lines, Bauer in 1955 suggested the ligature of the popliteal vein: the patients subjected to this treatment did not report severe complications, rather they improved their conditions, but nearly all of them relapsed due to the formation of side effects in turn refluent that rendered the ligature inefficient. Heart valve surgery instead has much more recent origins; it arose in fact due to Kistner who in 1968 executed the first human heart valve operation. This case was one of primary valve insufficiency. Kistner's operation, subsequently modified by others, still remains the operation commonly executed for this type of disease: after having identified with echo color doppler and phlebography the seat of a valve nest, where a non-functioning valve is present, an operation is conducted that provides for a direct access to the valve itself through a phlebotomy. The usually prolapsed valve flaps are stressed and stretched upward, in order to re-give them a correct morphology and a certain functionality. This is a delicate operation, to be conducted with great care. Most results are particularly encouraging and the healing of refractory trophic lesions occurs in most cases.
Nevertheless, the operation reconstructs only one valve within the venous axis, so that it is still necessary to employ complementary actions in post-operative stage (deambulation, elastic constriction, anti-edema devices), in order to maintain the homeostasis of the limb. If Kistner's operation is capable of correcting congenital valve disease, the same is however not applicable in the congenital forms of hypotrophy and atrophy and in the acquired forms of post-thrombotic type. In order to correct these forms, and in particular those of post-thrombotic type due to their high frequency, two operation types have been proposed: the first by Kistner in 1978, femoral transposition, and the other by Taheri in 1982, autograft.
Transposition is an interesting operation that consists of sectioning the superficial femoral vein and of anastomosis of the distal fragment on the deep femoral vein or on the saphena upstream of a competent valve.
This is a fascinating, well-functioning operation which, however, in over half the patients, cannot be conducted. The main reason consists of the fact that the deep femoral vein is also often incompetent in its first section and that the saphena is either incompetent or often absent due to previous removal.
The autograft proposed by Taheri consists of drawing, from the arm, a section of axillary vein which has a competent valve and its subsequent implant with end-end anastomosis in popliteal vein. It is necessary to ascertain beforehand that the axially vein contains a functioning valve; nevertheless, the size discrepancy and the reluctance of the patient to accept limitations of the upper limb render this operation one of rare application.
The creation of a neovalve by executing a parietal dissection is a further possibility, but this is a surgical operation executed in only a few centers and in limited numbers.
Research leading to a corrective action, when the above-described operations cannot be conducted, have led to the creation of various techniques, including: the reduction of the femoral vein size, the implant of cryopreserved valves and the implant of valves on stents.
The first two methods did not provide satisfactory results, while the implant of valves on stents is still in experimental phase.
In particular, the valves designed up to now have presented unsatisfactory experimental and clinical studies due to two main factors: malfunctioning of the valve and thrombosis in the valve nest.
Before commencing the description of the present invention, it should be observed that in physiology, the valve is not inserted within a cylindrical section having the same size at the valve itself, and above and below such valve, but rather it is inserted at a physiological expansion of the vessel, which is termed valve nest. The significance of this physiological expansion was recently well-documented by in vivo studies executed with B-Flow technique, which show that an ascending flow—through a decrease of speed thereof and consequent creation of return forces capable of acting in latero-medial direction on the valve—cause the closing of such valve. More clearly, the valves are situated in an intermediate position between the opening and closing and are closed by the same flow through forces that can only be exerted by virtue of the morphology of the valve nest itself.
None of the valve on stent designs made up to now have considered this important rule and therefore the valve does not work correctly, its valve nest is not washed by a flow, so that the valve remains in an immobile position and the valve nest precociously develops thrombosis.
At this point, there are essentially two possibilities foreseen for overcoming this great obstacle: creating a stent that reproduces the physiological expansion of the venous nest and inserting the new valve at this expansion, or overcoming the thrombosis and immobility in another manner. In particular the most important characteristics that a valve device must satisfy are the following: mobility (the valve must be movable and not fixed) and the fact that the valve nest, i.e. the nest comprised between the valve itself and the venous wall, is washed by the blood circulation. Since this washing action is exerted by the return of blood, and given that this return is not present since there is no expansion of the venous nest, the object of the present industrial invention patent application, described in detail hereinbelow, is to propose a new device that meets both the valve mobility requirement and the possibility of washing the valve nest.
It should be indicated that the valve mobility derives from opposite forces on the valve flap and given that there is no flow within the valve nest, the flap remains practically immobile, except for occasional forces that exert a counter-current flow. It is also opportune to specify that an anti-reflux action can be exerted by a valve mechanism formed by a valve with two flaps (bicuspid) or three flaps (tricuspid) or one flap (monocusp). The fact that the monocusp exerts an effective anti-reflux action, equal to a bicuspid or a tricuspid, has allowed attaining the definition of the present object, described hereinbelow.
It is important to specify that the valves are situated below the tributary veins, i.e. side branches that are inserted along the main venous axis and the main flow allows, during the speed slowing and accelerations thereof, the emptying of the tributary veins themselves through suction mechanisms and speed accelerations (Venturi effect). Physiological valves do not have tributary veins that arise directly from the venous nest whose washing, as stated above, is represented by the return circulation of the ascending flow. The awareness of these aspects has allowed designing and obtaining, with the aid of mathematical models, a valve device capable of restoring the physiological blood flow for use in treating the abovementioned diseases.